High strength magnesium alloy and method of production of the same

ABSTRACT

A high strength magnesium alloy improving the high temperature strength while not using any expensive rare earth elements and thereby reducing the cost and its method of production are provided, that is, a high strength magnesium alloy of a composition of the formula Mg 100−(a,b) Zn a X b  wherein X is one or more elements selected from Zr, Ti, and Hf, a and b are the contents of Zn and X expressed by at %, and the following relationships (1), (2), and (3): (1) a/28≦b≦a/9, (2) 2&lt;a&lt;10, (3) 0.05&lt;b&lt;1.0 are satisfied and comprising an Mg matrix phase in which Mg—Zn—X-based quasi-crystals and their approximant crystals are dispersed in the form of fine particles and a method of production of a high strength magnesium alloy comprising melting Mg in an inert atmosphere to form an Mg melt, adding Mg—Zn—X-based quasi-crystals (X being at least one of Zr, Ti, and Hf) into the Mg melt to form an alloy melt, casting the alloy melt, and heat treating the obtained casting to make the quasi-crystal and their approximant crystals precipitate in an Mg matrix phase.

TECHNICAL FIELD

The present invention relates to a high strength magnesium alloy, more particularly a magnesium alloy improved in high temperature strength and a method of production of the same.

BACKGROUND ART

Magnesium alloys are being used for various structural members due to their light weight. In particular, if used for automobiles, they are effective for improvement of the fuel economy and protection of resources and the environment.

As commercially available materials, as magnesium alloys for sand mold casting, ASTM AZ91C (standard composition [wt %]: Mg-8.7Al-0.7Zn-0.13Mn), ZE41A (same: Mg-4.2Zn-1.2RE-0.7Zr), etc. are in general use, while as wrought magnesium alloys, AZ61A (same: Mg-6.4Al-1.0Zn-0.28Mn), AZ31B (same: Mg-3.0Al-1.0Zn-0.15Mn), etc. are in general use.

Among these, the sand mold casting use alloys AZ91C and ZE41A are precipitation effect type alloys. By applying T6 (solution treatment+aging) or T5 (only aging) to the cast material, it is adjusted to the required strength. However, if exposed to room temperature or more, in particular 50° C. or more, for a long period of time, aging precipitation of the solid solution elements occurs, the alloy structure gradually changes, and, along with this, changes occur in the characteristics along with aging. As result, there was the defect that the thermal stability of the structure and characteristics was low and a stable, better high temperature strength could not be obtained.

Further, the wrought alloys AZ61A and AZ31B utilize crystal grain refinement through working and recrystallization at the time of rolling, extrusion, etc. as the reinforcement mechanism. However, if becoming a 100° C. or more high temperature, remarkable grain boundary slip distinctive to Mg occurs, so crystal grain refinement conversely becomes a cause of a drop in strength due to the increase in the grain boundary slip sites. Further, if exposed to a high temperature, the crystal grains grow and the effect of refinement is lost. This becomes a cause of a drop in the room temperature strength. As result, there was the defect that not only could not a high temperature strength be secured, but also the room temperature strength became thermally unstable.

To counter the defects of the conventional commercially available materials and secure a better high temperature strength, Japanese Patent Publication (A) No. 2002-309332 discloses a Mg-1 to 10 at % Zn-0.1 to 3 at % Y alloy with a solid solution matrix strengthened by dispersion of quasi-crystal grains. The cast structure is one where eutectic structures of quasi-crystals are formed at the α-Mg crystal grain boundaries and where the quasi-crystals are finely and uniformly dispersed by hot working. Quasi-crystals are far higher in hardness than similar compositions of crystalline compounds, so magnesium superior in strength and ductility can be obtained. However, while the thermal stability was high, the strength itself became an extent equal to the commercially available alloy of a similar composition like ZE41. There was the limit that a further high temperature strength could not be obtained.

As opposed to this, Japanese Patent Publication (A) No. 2005-113235 discloses, as an alloy improving the high temperature strength, a magnesium alloy of a composition of Mg_(100−(a+b))Zn_(a)Y_(b) satisfying a/12≦b≦a/3 and 1.5≦a≦10 and comprising an Mg matrix phase in which Mg₃Zn₆Y₁ quasi-crystals and their approximant crystals (complex structural phases derived from quasi-crystals) are dispersed in the form of fine particles.

Furthermore, Japanese Patent Publication (A) No. 2006-89772 discloses a magnesium alloy of a composition of Mg_(100−(a+b+c))Zn_(a)Zr_(b)Y_(c) satisfying a/12≦(b+c)≦a/3 and 1.5≦a≦10 and 0.05<b<0.25c and comprising an Mg matrix phase in which fine particles of approximant crystals are dispersed.

Further, Japanese Patent Publication (A) No. 2005-113234 discloses a magnesium alloy of a composition of Mg_(100−(a+b+c))Zn_(a)Al_(b)Y_(c) satisfying (a+b)/12≦c≦(a+b)/3 and 1.5≦a≦10 and 0.05a<b<0.25a and comprising an Mg matrix phase in which Mg₃Zn₆Y₁ quasi-crystals and their approximant crystals (complex structural phases derived from quasi-crystals) are dispersed in the form of fine particles.

The magnesium alloys of the above prior arts all realize better high temperature strengths by dispersing quasi-crystals and their approximant crystals as fine reinforcing particles in the Mg matrix phase, but had the problem that a rise in cost was unavoidable since a rare earth element (Y) was an essential ingredient.

DISCLOSURE OF THE INVENTION

The present invention has as its object the provision of a high strength magnesium alloy improving the high temperature strength while not using any expensive rare earth elements and thereby reducing the cost and its method of production.

To achieve that object, the high strength magnesium alloy of the first aspect of the invention has a composition of the formula Mg_(100−(a+b))Zn_(a)X_(b) wherein X is one or more elements selected from Zr, Ti, and Hf, a and b are the contents of Zn and X expressed by at %, and the following relationships (1), (2), and (3):

a/28≦b≦a/9   (1)

2<a<10   (2)

0.05<b<1.0   (3)

are satisfied and

comprising an Mg matrix phase in which Mg—Zn—X-based quasi-crystals and their approximant crystals are dispersed in the form of fine particles.

A method of the second aspect of the invention for producing the high strength magnesium alloy of the first aspect of the invention includes:

a step for melting Mg in an inert atmosphere to form an Mg melt,

a step of adding Mg—Zn—X-based quasi-crystals (X being at least one of Zr, Ti, and Hf) into the Mg melt to form an alloy melt,

a step of casting the alloy melt, and

a step of heat treating the obtained casting to make the quasi-crystals and their approximant crystals precipitate in an Mg matrix phase.

The high strength magnesium alloy of the present invention is comprised of a Mg matrix phase in which fine particle-shaped Mg—Zn—X-based quasi-crystals and their approximant crystals are dispersed as reinforcing particles whereby high strength, in particular high temperature strength equal to an alloy in which quasi-crystals and approximant crystals are dispersed using conventional rare earth elements, can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photo of an electron beam diffraction pattern of a Mg₁₁Zn₈₃Zr₆ quasi-crystal prepared by the present invention.

FIG. 2 is a transmission electron micrograph showing the metal structure of an Mg alloy prepared by the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The high strength magnesium alloy of the present invention, by setting the above range of composition, can be given a structure comprised of an Mg matrix phase in which quasi-crystals comprised of Mg—Zn—X (X=one or more of Zr, Ti, and Hf) and their approximant crystals are dispersed.

“Quasi-crystals” are compounds of a structure with a regular structure in a short range (typically a fivefold symmetry), but without the translational symmetry characterizing ordinary crystals. As compositions forming quasi-crystals, up until now Al—Pd—Mn, Al—Cu—Fe, Cd—Yb, Mg—Zn—Y, etc. have been known. Being special structures, compared with crystalline intermetallic compounds of similar compositions, they have various special properties such as high hardness, high melting point, low coefficients of friction, etc.

“Approximant crystals” are crystalline compounds having complex structures derived from quasi-crystals and partially having structures similar to those quasi-crystals. They have properties similar to the original quasi-crystals.

The Mg—Zn—X-based quasi-crystals have compositions of preferably, by at % ratio of Zn and X, a:b=83:6 and are one or more crystals selected from Mg₁₁Zn₈₃Zr₆, Mg₁₁Zn₈₃Ti₆, and Mg₁₁Zn₈₃Hf₆. However; the invention does not have to be limited to these. Quasi-crystals of compositions allowed within the above prescribed range of alloy composition and their approximant crystals are also allowed. The fine particles have sizes of several tens of nm to several hundreds of nm.

Quasi-crystals and their approximant crystals are extremely hard and remain stable without breaking down up to about 230° C., so strongly interact with the dislocations when dispersing as fine particles in the Mg matrix phase, exhibit an extremely high dispersion reinforcing action, and improve the ordinary temperature and high temperature strength. In particular, the fine particles present at the α-Mg crystal grain boundaries suppress the grain boundary slip at a high temperature and contribute to better high temperature strength.

The alloy of the present invention uses Zr, Ti, or Hf instead of the rare earths of the conventional alloys as a component ingredient of the quasi-crystals and their approximant crystals. These elements have a hard time dissolving in the melt of the main ingredient Mg of the alloy, so it is not possible to produce the alloy by directly producing the alloy melt of the final composition like with the conventional alloys including rare earth elements. That is, even if weighing out the materials of the ingredients of the alloy in accordance with the final alloy composition, charging them together in a melting furnace, and heating them to form the melt, the high melting point Zr, Ti, or Hf will not dissolve in the melt and will end up remaining as a solid. These high melting point elements have melting points of extremely high temperatures or higher than even the melting point of Mg.

The Y and other rare earth elements used in conventional alloys have far higher melting points than Mg, but when contacting the Mg melt formed earlier at a low temperature at the time of melting, they form alloys and become lower in melting point so easily dissolve in the Mg melt. Therefore, conventional alloys could be obtained by directly producing melts of the final compositions.

The alloy of the present invention, in the above way, cannot be obtained by directly producing an alloy melt of the final composition, so in the method of the present invention, only Mg is melted to form an Mg melt, then the quasi-crystals are added to this to enable the formation of the alloy melt. The final alloy composition is adjusted to by the amount of the Mg melt and the composition and amounts of addition of the quasi-crystals. The alloy melt is sufficiently stirred to obtain a uniform melt.

The obtained alloy melt is cast by an ordinary method. The obtained casting is heat treated to make the quasi-crystals and their approximant crystals precipitate as fine particles in the Mg matrix phase.

Due to this, finally, a structure of an Mg matrix phase in which fine particles of quasi-crystals and their approximant crystals are dispersed is obtained.

The mechanism for the production of the fine particles of the quasi-crystals and their approximant crystals has not been fully elucidated at the present point of time, but is believed to be as follows:

An alloy melt comprised of an Mg melt into which quasi-crystals have been added and sufficiently stirred may appear visually to be uniform, but microscopically there are variations in composition in the melt. Fine regions where specific elements are segregated are distributed across the entire region of the melt. In the cooling process at the time of casting, quasi-crystals and approximant crystals or their precursor compositions finely precipitate around these fine segregated regions. The solidified casting is comprised of an Mg matrix phase in which the component elements Zn and X (at least one of Zr, Ti, and Hf) of-the quasi-crystals or approximant crystals are dissolved in a supersaturated state. The quasi-crystals or approximant crystals precipitate as fine particles around the fine precipitates due to heat treatment.

That is, in the metal structure of the finally obtained alloy, fine particles of the quasi-crystals and approximant crystals formed in the cooling process at the time of casting and fine particles of the quasi-crystals and approximant crystals precipitating in the subsequent heat treatment are present. The two both contribute to improvement of the strength by dispersion strengthening.

Examples Example 1

According to the present invention, Mg—Zn—Zr magnesium alloys of compositions giving the final compositions of the alloys as a whole as shown in Table 1 were prepared and examined for metal structure and subjected to tensile tests.

(1) Preparation of Quasi-Crystals

Metals of pure Mg (99.9%), pure Zn (99.99%), and pure Zr (99.5%) (figures in parenthesis indicate purity) were weighed out to give a quasi-crystal composition, by at % ratio, of Mg₁₁Zn₈₃Zr₆. A total amount of 5 g was set in an alumina Tammann tube (φ12 mm×10 mm) which was then sealed in a quartz tube. The inside of the quartz tube was replaced with high purity argon.

This was raised in temperature from room temperature to 700° C. over 5 hours, was held there for 12 hours, then was lowered in temperature to 500° C. and was held there for 48 hours. Due to this, Mg₁₁Zn₈₃Zr₆ quasi-crystals were obtained. As shown by the electron beam diffraction pattern in FIG. 1, typical fivefold symmetry was confirmed.

The obtained clumps of quasi-crystals were crushed to obtain particles of a size of several tens of μm which were then used for the following.

(2) Production of Alloy

Pure Mg (purity 99.99%) was charged into a graphite crucible and raised in temperature to 700° C. in a high frequency melting furnace held in an argon atmosphere so as to melt it and form an Mg melt.

Into the Mg melt, the quasi-crystals prepared in the above (1) were added in an amount adjusted to give a final alloy composition of the Mg—Zn—Zr composition shown in Table 1, then the melt was stirred until the crystals completely dissolved and the melt became uniform while holding the melt temperature at 700° C., then was held in that state for 2 hours.

(3) Casting

The obtained alloy melt was held at 700° C. and cast into a cast iron JIS No. 4 boat type casting mold (70 mm×70 mm×300 mm) preheated to about 100° C.

(4) Heat Treatment

The obtained casting was heat treated in an argon atmosphere at 500° C. for 48 hours.

<Observation of Metal Structure>

The heat treated sample was observed for metal structure by a transmission electron microscope (TEM).

As result, as shown in FIG. 2, fine precipitates of several tens of nm size were observed in white spots in the α-Mg crystal grains. The precipitates were confirmed to be Mg₁₁Zn₈₃Zr₆ quasi-crystals and their approximant crystals.

<Tensile Test>

A rod-shaped tensile test piece with parallel parts of φ5×25 mm was taken from the heat treated sample and subjected to a tensile test at room temperature, 150° C., and 200° C. The test was conducted using an AG-250 kND tensile tester made by Shimadzu at a tensile rate of 0.8 mm/min.

Furthermore, the heat treated, then extruded sample was also similarly subjected to a tensile test. The extrusion conditions were an extrusion temperature of 250° C. and an extrusion ratio of 10:1.

Further, the comparative examples with compositions outside the scope of the present invention were also similarly subjected to a tensile test.

Example 2

According to the present invention, Mg—Zn—Ti magnesium alloys of compositions giving final compositions of the alloys as a whole as shown in Table 1 were prepared and examined for metal structure and subjected to tensile tests.

Except for preparing, as quasi-crystals, Mg₁₁Zn₈₃Ti₆ quasi-crystals and adding them to a pure Mg melt, the same procedure was used as in Example 1 for preparation.

After heat treatment, using a transmission electron microscope, fine precipitates of several tens of nm size were observed in white spots in the α-Mg crystal grains. The precipitates were confirmed to be Mg₁₁Zn₈₃Ti₆ quasi-crystals and their approximant crystals.

The heat treated samples and samples extruded under the same conditions as in Example 1 were subjected to tensile tests under the same conditions as in Example 1.

Further, the comparative materials with compositions outside the scope of the present invention were also subjected to similar tensile tests.

Furthermore, the conventional materials were also subjected to similar tensile tests.

The above test results are shown together in Table 1.

TABLE 1 Tensile strength (MPa) Elongation (%) Method of Room Room Class production Composition (at %) temperature 150° C. 200° C. temperature 150° C. 200° C. Mg Zn Zr Present Casting 94.59 5.05 0.36 200 183 156 4 22 26 invention 96.82 3.02 0.16 206 189 161 3 14 22 (Example 1) 91.24 8.04 0.72 201 193 163 3 11 18 Extrusion 97.67 2.24 0.09 225 180 150 6 25 27 Comparative Casting 95.35 4.52 0.13 187 135 108 5 23 32 material Mg Zn Ti Present Casting 94.56 5.07 0.37 203 193 152 4 21 24 invention 96.79 3.04 0.17 204 185 163 3 15 21 (Example 2) 91.23 8.03 0.74 209 195 165 2 12 16 Extrusion 91.23 8.03 0.74 320 258 235 21 24 30 Comparative Casting 91.62 8.15 0.23 190 152 126 2 15 27 material Conventional Casting AZ91C-T6 275 185 140 5 31 33 material ZE41A-T5 205 165 130 5 15 29 Extrusion AZ61A-F 325 225 150 16 40 42 AZ31B-F 275 170 110 12 39 42

The present invention materials are particularly superior in tensile strength at 150° C. compared with the conventional materials. Further, the drop in strength accompanying a rise in temperature from room temperature to 150° C. is extremely small. This is because the fine particles comprised of the quasi-crystals and their approximant crystals precipitated and dispersed in the α-Mg crystal grains have extremely high thermal stabilities, so strongly interact with the dislocations even under a high temperature of 150° C. and effectively function as barriers to movement of the dislocations. The comparative materials of the compositions outside the scope of the present invention either do not form quasi-crystals and their approximant crystals or even if forming them form them in extremely fine amounts, so almost no dispersion strengthening action is obtained due to these formed phases and a high strength cannot be obtained.

INDUSTRIAL APPLICABILITY

According to the present invention, there are provided a high strength magnesium alloy improving the high temperature strength while not using any expensive rare earth elements and thereby reducing the cost and its method of production. 

1. A high strength magnesium alloy of a composition of the formula Mg_(100−(a+b))Zn_(a)X_(b) wherein X is one or more elements selected from Zr, Ti, and Hf, a and b are the contents of Zn and X expressed by at %, and the following relationships (1), (2), and (3): a/28≦b≦a/9   (1) 2<a<10   (2) 0.05<b<1.0   (3) are satisfied and comprising an Mg matrix phase in which Mg—Zn—X-based quasi-crystals and their approximant crystals are dispersed in the form of fine particles.
 2. A high strength magnesium alloy as set forth in claim 1, wherein said quasi-crystals are at least one type selected from Mg₁₁Zn₈₃Zr₆, Mg₁₁Zn₈₃Ti₆, and Mg₁₁Zn₈₃Hf₆.
 3. A method of production of a high strength magnesium alloy as set forth in claim 1, including: a step for melting Mg in an inert atmosphere to form an Mg melt, a step of adding Mg—Zn—X-based quasi-crystals (X being at least one of Zr, Ti, and Hf) into said Mg melt to form an alloy melt, a step of casting said alloy melt, and a step of heat treating the obtained casting to make said quasi-crystals and their approximant crystals precipitate in an Mg matrix phase.
 4. A method of production of a high strength magnesium alloy as set forth in claim 3, further comprising preparing said Mg—Zn—X-based quasi-crystals by: a step of weighing the Mg, Zn, and X materials to give a quasi-crystal composition, a step of charging said weighed materials in a crucible and melting them in an inert atmosphere to form a melt, a step of lowering a temperature of said melt and holding it at a single phase region where only said quasi-crystals are present, and a step of cooling from the temperature of said single phase region to room temperature. 